The present invention relates to a method of acquiring an error correction value of a reference frequency in a terminal device capable of locating a position using a satellite positioning system (SPS), a terminal device, and a recording medium.
A positioning system has been used in practice which locates the present position of a GPS receiver utilizing an SPS such as a global positioning system (GPS) (e.g. JP-A-2000-131415 (FIG. 1 etc.)).
The GPS receiver receives signals (hereinafter called “satellite signals”) from four GPS satellites positioned in the sky, for example, and calculates the distance between each GPS satellite and the GPS receiver (hereinafter called “pseudo-range”) based on the difference between the time at which the satellite signal is transmitted from each GPS satellite and the time at which the satellite signal reaches the GPS receiver (hereinafter called “delay time”). The GPS receiver calculates the present position of the GPS receiver using the position of each GPS satellite in the satellite orbit and the pseudo-range. The satellite signal includes a coarse/access (C/A) code and information such as a navigation message.
The GPS receiver may calculate the pseudo-range utilizing the code phase of the C/A code carried by the satellite signal. The C/A code is a signal having a bit rate of 1.023 Mbps and a bit length of 1023 bits (=1 msec=300 km). If the initial position of the GPS receiver is known within an error range of 150 km, the number of C/A codes existing between each GPS satellite and the GPS receiver can be estimated. Therefore, the pseudo-range can be calculated by utilizing the fraction portion (code phase) of the C/A code.
In order to receive the satellite signal, the GPS receiver calculates the search center frequency taking into account the carrier frequency of the satellite signal and the Doppler shift caused by the relative movement of each satellite and the GPS receiver. The GPS receiver determines a range with a specific width around the search center frequency to be a search band (hereinafter called “search range”).
The GPS receiver down-converts the frequency of the received satellite signal using a clock signal from a reference oscillator (hereinafter called “local oscillator”).
However, the local oscillator undergoes a drift. The term “drift” refers to a change in oscillation frequency due to a change in temperature.
When the reception frequency after down-conversion falls outside the search range due to the drift of the local oscillator, the search efficiency may be decreased, or it may be impossible to search for the satellite.
In order to deal with this problem, technology has been proposed which acquires a reference frequency with a high accuracy from a communication base station or the like, and uses the reference frequency to determine the search range (e.g. JP-T-2000-506348, JP-A-2002-228737, and JP-T-11-513787).
However, the reference frequency acquired from the communication base station or the like has an error even through the reference frequency has a high accuracy. The accuracy of the reference frequency is generally lower than the accuracy of the carrier frequency of the satellite signal. Therefore, when calculating the drift using the reference frequency, the search range must be increased for the error of the reference frequency, whereby the search range becomes wide.